Ok--I'll take a shot at this one. This is a broad question and I am going on the assumption that this is concerning a street type car. Generally speaking, the factory utilizes a relatively heavy flywheel. This is to make the car operate smoother. The heavy flywheel does take a little more HP to turn it up, BUT the car takes advantage of this heavier flywheel. A heavier flywheel stores more energy than a light one. This energy helps keep the car from bogging down on takeoff and in between gears. The weight keeps the momentum of the engine/flywheel up and it doesn't decelerate as quickly. If this momentum wasn't there, the car would be more "herky=jerky". Drag racers use lightweight flywheels because they don't take as much HP to spin up, BUT the racers don't need smoothness. They leave under full throttle and shift gears as quickly as possible.. They have alot more horsepower and less vehicle weight to move. Generally speaking, in most street/strip applications, an automatic, (with stall convertor and shift kit) is quicker. It shifts quicker and has one less shift in the quarter mile. Of course there are exceptions to this generality. Hope this helps a little.
..............Oldschool
